The present invention relates to demodulators of quadrature amplitude modulation (QAM) digital radio communications systems, and more specifically to the improvement of the dc offset of control means of such a demodulator which controls the dc offset of the quadrature signals which are derived from the demodulation of the incoming signal by synchronous detection.
In the past, amplitude and phase shift keying techniques have been developed for use in digital radio communications systems to take advantage of their high efficiency in frequency utilization. Among the amplitude and phase shift keying techniques, multilevel quadrature amplitude modulation technique has been extensively used because of its advantages for practical applications. The sinewave carrier used in the multilevel quadrature amplitude modulation systems is modulated so that its amplitude and phase varies independently of each other as a function of the two baseband signals. In the signal space diagram which is a polar coordinate representation of the quadrature amplitude modulation signal, the signal points of such QAM signals are located at intersections of rows and columns in an area bounded in a rectangular configuration.
In the phasor diagram of a conventional 256-QAM signal, the 256 signal points of the QAM signal align themselves along I- and Q-channel axes, forming a square shaped constellation of 16 rows and 16 columns. Another multilevel QAM technique that has recently been developed and is known as stepped square quadrature amplitude modulation (SS-QAM) is one having a phasor diagram having a stepped square signal constellation of 18 rows and 18 columns. With this stepped square phasor diagram, the symbol error rate can be improved over the conventional-QAM system, or C-QAM. A further benefit of the SS-QAM system is that it is tolerant of the nonlineary characteristic of the transmitter's high gain amplifier. A comparison between a 256-SS-QAM modulation system and a 256-C-QAM system reveals that a group of 6 signal points located at each corner area of the square shaped constellation of the 256-C-QAM system corresponds to one of two rows and two columns of 6 signal points each which are located on the imaginary outlines of the stepped square configuration of the 256-SS-QAM system.
A demodulator of the 256-SS-QAM system has been proposed to combine the benefits of the improved transmission characteristics with low-cost circuits employed in the C-QAM system. One approach is to perform transposition of such outermost signals of the SS-QAM signal constellation to corresponding signal points of the C-QAM system to control the dc offset of the demodulated quadrature signals. As shown in FIG. 1, the prior art demodulator of a 256-SS-QAM system comprises a signal point transposer 104 for each of the I-channel (in-phase) and the Q-channel (quadrature-phase) systems. Signal point transposer 104i of the I-channel receives digital signals supplied from A/D converters 103i and 103q and transposes the signal points of the I-channel signal which lie on the outermost columns of the stepped square signal constellation of the SS-QAM system to the corresponding signal points of the C-QAM system and generates an I-channel main data signal Di which represents one of 16 amplitudes of the I-channel signal. Transposer 104i further generates an I-channel error signal Ei which represents an error contained in the I-channel main data signal Di. The I-channel error signal Ei is a binary signal having one-half the quantum size of the 16 quantization levels of the I-channel main data signal Di and represents the direction of deviation of each of the 16 amplitude levels of the I-channel signal with respect to a corresponding one of the prescribed 16 decision thresholds with which the A/D converter 103i compares the amplitude of the I-channel signal demodulated by synchronous detector 100 to convert it to a corresponding digital signal. The error signal Ei having a logic-0 indicates that the signal point of the I-channel main data signal Di deviates on the outer side of the corresponding signal point and hence it deviates on the negative side of the corresponding decision threshold and the error signal having a logic-1 indicates that signal Di deviates on the inner side of the corresponding point and hence on the positive side of the corresponding decision threshold.
The I-channel main data signal Di and the I-channel error signal Ei from the signal point transposer 104 are supplied to a control signal generator 102, which identifies those signals which are located on the outside of the I-axis 15-th column of the C-QAM phasor diagram, where errors of positive peak amplitudes exist, and further identifies those signals which are located on the outer side of the I-axis 0-th column, where errors of negative peak amplitudes exist. The signal points falling outside the 15-th column of the C-QAM system phasor diagram can be considered to be a deviation of the amplitude of the signal on the I-axis 15-th column on the positive side of the optimum amplitude and the signal points falling outside the 0-th column of the C-QAM system can be considered to be a deviation of the amplitude of the signal at the I-axis 0-th column on the negative side of the optimum amplitude. Control signal generator 102 counts such positive and negative deviation that occur within a prescribed time interval and compares the counts to control the automatic gain controller 101 so that it reduces the dc offset when the count of the positive deviations is greater than the other by a predetermined amount and increases it when the count of the negative deviations is greater than the other by a predetermined amount. In this way, the I-channel main data signal Di represents the true amplitude level of the I-channel signal. Similar operations take place in the Q-channel. Signal point transposer 104q of the Q-channel receives digital signals supplied from A/D converters 103q and 103i and transposes the signal points of the Q-channel signal which lie on the outermost rows of the stepped square signal constellation of the SS-QAM system to the corresponding signal points of the C-QAM system and generates a Q-channel main data signal Dq which represents one of 16 amplitudes of the Q-channel signal.
However, four peak amplitudes at signal points a, b, c and d of the SS-QAM system as indicated in FIG. 4 are transposed respectively to signal points a', b', c' and d' of the C-QAM system shown at FIG. 5. Since these transposed signal points do not contribute to the generation of the dc offset signal, the error signal generated by the prior art SS-QAM demodulator lacks precision. A further disadvantage is that the signal point transposition requires complex, expensive circuitry.